The imaginary unit is the square root of negative 1. A lot of people like
to call this the "definition" of , but the definition of df-sqr11981 has already been crafted with being mentioned
explicitly,
and in any case it doesn't make too much sense to define a value based on
a function evaluated outside its domain. A more appropriate view is to
take ax-i2m19005 or i211422 as the "definition", and
simply postulate the
existence of a number satisfying this property. This is the approach we
take here. (Contributed by Mario Carneiro, 10-Jul-2013.)

Convert between three-quantifier and four-quantifier versions of the
Cauchy criterion. (In particular, the four-quantifier version has no
occurence of in
the assertion, so it can be used with rexanuz12090
and friends.) (Contributed by Mario Carneiro, 15-Feb-2014.)